1. Field of the Invention
The present invention relates to the switching of electric coils used to emit and receive radio frequency energy, in particular in the field of magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR).
This invention can be used in conjunction with one, some or all of the inventions described in our simultaneously filed applications entitled Magnetic Resonance Imaging of Fluid Flows, Phase Error Correction in Magnetic Resonance Imaging, Magnetic Field Measurement and Gradient Coils in Magnetic Resonance Imaging Machines.
2. The Prior Art
In MRI, a subject, for instance a live patient, is placed in a particular magnetic field and the hydrogen nuclei within the subject are excited by way of radio frequency (RF) energy. By sensing RF energy received back from the subject to detect resonances of the nuclei it is possible, given accurate knowledge of the magnetic field prevailing in the three-dimensional volume which contains the subject, to construct three-dimensional images of the internal structure of the subject.
To operate such apparatus to obtain proper images it is important firstly to be able to know accurately what the magnetic field is across the subject, and it is a well known technique to utilize magnetic field gradients so that the magnetic field varies constantly across the three-dimensional volume in question.
To perform the RF excitation and sensing mentioned above RF transmit and receive coils are provided. Both types of coil are tuned to the same frequency, i.e. that of the nuclear magnetic resonance. The transmit coil normally surrounds the patient, and the receive coil is placed over the region of the patient from which images are required, e.g. around the chest for heart imaging. The transmit coil and receive coil therefore have a large mutual inductance and the two resonant circuits are thereby coupled to each other and the resonance frequencies are shifted due to the coupling.
The amount and direction of the shift is quite unpredictable, depending on position, orientation, precise tuning and matching, patient size etc. making it practically impossible to use two such coils in this situation, unless the receiver coil is so small that the coupling is negligible.
In some previous systems, it has been possible to orient the two coils at right angles to each other to minimise the coupling, but this method cannot be used in particular if the transmitter coil is a quadrature coil. (A quadrature coil is more efficient than a single coil because it consists of two mutually orthogonal coils taking advantage of the minimised coupling between them. It generates a more powerful transmit R.F. field (with circular polarization) than a single coil (with either horizontal or vertical polarization).)
To remove the coupling effects the resonance of the unused coil is disabled at each stage in the imaging process. During the R.F. excitation of the NMR signal only the transmitter coil is enabled. During the data acquisition, only the receiver coil is enabled. The switching should be rapid, e.g. 100 microseconds or less, and reliable with feedback preventing the R.F. transmitter from delivering high R.F. power levels into the transmitter coil if the coil's resonance is disabled.
It is thus necessary to switch the various RF coils in and out of operation at various times during an imaging process, but it is also important to do this in such a way that the magnetic field, which is important for the imaging process, is not affected. For this reason the use of electrically operated switches to switch the RF coils can cause difficulties because they require the use of electric currents to operate them and if these are introduced into the vicinity of the subject they can affect the magnetic field around the subject.
The most common problem associated with conductors bringing such currents into the imaging volume is that they may bring in noise and unwanted signals (for example, short-wave radio stations) at the frequency of the NNR signal, which degrades the quality of the images. Conductors may carry in external noise and signals into the imaging volume, which is carefully shielded against such signals typically using a Faraday cage. This problem is especially difficult for the receive R.F. coil, which is the most sensitive part of the system. Conventional R.F. coil switching uses PIN diodes switched on by an external D.C. current. The cable carrying the D.C. PIN diode energisation signal must be carefully filtered to stop any signal at the NMR signal frequency, which filtering adds to the complexity of the overall apparatus.